Under certain operating conditions, engines that have high compression ratios, or are boosted to increase specific output, may be prone to low speed pre-ignition combustion events. The early combustion due to pre-ignition can cause very high in-cylinder pressures, and can result in combustion pressure waves similar to combustion knock, but with larger intensity. Strategies have been developed for prediction and/or early detection of pre-ignition based on engine operating conditions. Additionally, following detection, various pre-ignition mitigating steps may be taken.
For example, in response to the occurrence of a cylinder pre-ignition event, the affected cylinder or cylinder bank may be enriched for a defined number of combustion events. However, under the same operating conditions, some cylinders of an engine may have a higher incidence of pre-ignition than others. The higher pre-ignition rate may cause the affected cylinders to degrade earlier, thereby affecting engine performance.
Thus in one example, the above issue may be at least partly addressed by a method of operating an engine. In one embodiment, the method comprises, fueling each cylinder of the engine based on a pre-ignition count of each cylinder to bring each cylinder to a common pre-ignition count while maintaining an exhaust air-to-fuel ratio of the engine at or near stoichiometry.
In one example, an engine control system may compare the pre-ignition counts of each engine cylinder. Then, based on the comparison, each cylinder may be fueled to bring the pre-ignition count of each cylinder closer to each other, for example, to a common pre-ignition count. As an example, an engine cylinder with a relatively higher pre-ignition count (such as a pre-ignition count higher than the common pre-ignition count) may be enriched while an engine cylinder with a relatively lower pre-ignition count (such as a pre-ignition count lower than the common pre-ignition count) may be enleaned. The degree (e.g., amount, duration, etc.) of enrichment and enleanment may be based on the number of cylinders having a pre-ignition count different from the common count (and in which direction), as well as the deviation of each cylinder's pre-ignition count from the common pre-ignition count. Further, the amounts may be adjusted so that the exhaust air to fuel ratio of the engine is maintained at or near stoichiometry.
In this way, the fueling of each cylinder can be adjusted to balance the occurrence of pre-ignition in each cylinder, thereby reducing a high occurrence of pre-ignition in any given cylinder. At the same time, while balancing the cylinder pre-ignition counts, an air-to-fuel ratio of the engine exhaust can be maintained at stoichiometry. In this way, engine degradation due to pre-ignition can be reduced while improving engine fuel economy and exhaust emissions.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.